Grease composition for electrical contacts

ABSTRACT

The present invention provides a grease composition for electrical contacts, including: a thickener; a base oil; and an organic sulfur compound, wherein the organic sulfur compound is a compound having no heterocycle containing nitrogen in a molecule thereof.

TECHNICAL FIELD

The present invention relates to a grease composition for electrical contacts having excellent wear resistance of gold plating.

BACKGROUND ART

As electrical contacts and connectors used in electrical products, OA equipment, in-vehicle electrical components, various types of facility equipment, and the like, contact materials are used in which plating of gold or a gold alloy, which has excellent conductivity, oxidation resistance, and corrosion resistance among metals, is applied to the base metal of copper or a copper alloy to ensure good conductivity.

In recent years, many mechanical parts are required to be reduced in cost, and the electrical contacts, which are a part of them, are also naturally required to be reduced in cost. One of the methods of reducing the cost of electrical contacts is to reduce the thickness of gold plating to reduce the amount of gold used, which is a factor in increasing the cost. However, gold is subject to adhesion and wear due to sliding. When the thickness of the gold plating is reduced, there is more concern than before about the exposure of the base metal due to the progress of wear, the subsequent oxidation of the base metal, and the poor conductivity.

As a countermeasure, the life of the electrical contact can be improved by using a lubricant having excellent wear resistance of gold plating. As a lubricant for reducing the wear of gold plating, a lubricant added with mercaptobenzothiazole or dibenzothiazyl disulfide has been reported (Patent Literature 1).

CITATION LIST Patent Literature

[Patent Literature 1] Japanese Patent Application Publication No. 2005-068391

SUMMARY OF INVENTION Technical Problems

However, in order to meet the demand for further thinning of gold plating, it is further required to improve the wear resistance of gold plating. Patent Literature 1 states that thickening can prevent oil from dripping from the plating surface, but when the viscosity of the oil decreases under high temperature, there is still a concern that the oil will drip and stain the peripheral members.

From the above, an object to be achieved by the present invention is to provide a grease composition for electrical contacts in which the wear resistance of gold plating is improved more than before while preventing the peripheral members from being stained due to the dripping of the lubricant.

Solution to Problems

In response to the above-mentioned object of improving wear resistance, the present inventors have improved the wear resistance of gold plating more than before by using an organic sulfur compound containing no heterocyclic nitrogen.

The present invention makes it possible to provide a grease composition having improved wear resistance of gold plating. According to the grease composition of the present invention, it is possible to improve the life of an electrical contact having a small thickness of gold plating.

1. A grease composition for electrical contacts, comprising a thickener, a base oil, and an organic sulfur compound, wherein the organic sulfur compound is a compound having no heterocycle containing nitrogen in a molecule thereof. 2. The grease composition according to item 1, wherein the organic sulfur compound is at least one selected from the group consisting of thiols, disulfides, alkylthiophenes, sulfides of oils or fats, sulfides of olefins, and thiophosphoric acid or esters thereof. 3. The grease composition according to item 1, wherein the organic sulfur compound is at least one selected from the group consisting of thiols. 4. The grease composition according to item 1, wherein the organic sulfur compound is a thiol having a linear or branched alkyl group having 6 to 24 carbon atoms. 5. The grease composition according to item 1, wherein the organic sulfur compound is stearyl mercaptan. 6. The grease composition according to any one of items 1 to 5, wherein the organic sulfur compound is contained in an amount of about 0.001 to 5% by mass based on the total mass of the composition. 7. The grease composition according to any one of items 1 to 6, wherein the base oil is a synthetic hydrocarbon oil. 8. The grease composition according to any one of items 1 to 7, wherein the base oil is poly-alpha-olefin. 9. The grease composition according to any one of items 1 to 8, wherein a kinematic viscosity of the base oil at 40° C. is about 200 mm²/s or less. 10. The grease composition according to any one of items 1 to 9, wherein the base oil is contained in an amount of about 40 to 97% by mass based on the total mass of the composition. 11. The grease composition according to any one of items 1 to 10, wherein the thickener is at least one selected from the group consisting of Li soaps, urea compounds, polytetrafluoroethylene, inorganic fine particles, and calcium sulfonate complex. 12. The grease composition according to any one of items 1 to 11, wherein the thickener is polytetrafluoroethylene. 13. The grease composition according to any one of items 1 to 12, wherein the thickener has a primary particle diameter of about 80 to 400 nm. 14. The grease composition according to any one of items 1 to 13, wherein the thickener is contained in an amount of about 2 to 59% by mass based on the total mass of the composition. 15. The grease composition according to any one of items 1 to 14, further comprising at least one additives selected from the group consisting of antioxidants, rust inhibitors, metal corrosion inhibitors, oiliness improvers, and viscosity index improvers. 16. The grease composition according to item 15, wherein the additive is contained in an amount of about 0.01 to 20% by mass based on the total mass of the composition. 17. The grease composition according to any one of items 1 to 16, wherein the composition has a 60-stroke worked penetration measured according to JIS K2220 7 of about 220 to 400. 18. The grease composition according to any one of items 1 to 17, wherein a material of the electrical contacts is gold or a gold alloy.

DESCRIPTION OF EMBODIMENTS <Thickener>

The thickener used in the grease composition of the present invention is not particularly limited. Specific examples include soap-based thickeners typified by Li soaps such as lithium stearate and lithium 12-hydroxystearate, and Li complex soaps; urea-based compounds typified by diurea compounds; organic thickeners typified by polytetrafluoroethylene (PTFE) and sodium terephthalate; inorganic fine particles typified by organic bentonite and silica; and calcium sulfonate complex.

Among these, one or more selected from the group consisting of Li soaps, urea compounds, PTFE, inorganic fine particles, and calcium sulfonate complex are preferable.

More preferable are PTFE, silica, diurea compounds, and calcium sulfonate complex. These can withstand use at high temperatures because the thickener can remain solid even at high temperatures due to its high melting point. In particular, PTFE 15 preferable.

PTFE has a primary particle diameter of preferably about 80 to 400 nm, and more preferably about 100 to 200 nm, from the viewpoint of having an appropriate thickening ability. In addition, those produced by the direct polymerization method are preferable. As a result, the grease can be kept in a semi-solid state. Note that in the present specification, the primary particle size means the average particle size of the primary particles (that is, it refers to the basic constituent particles of the substance, and those that are formed by aggregation of primary particles and behave as one particle are called secondary particles), and refers to the arithmetic mean value of the values measured with an electron microscope.

Silica has a specific surface area of preferably about 15 to 410 m²/g from the viewpoint of having an appropriate thickening ability. Note that in the present specification, the specific surface area refers to a value measured by the BET method.

The diurea compound is represented by the following formula (I)

R1-NHCONH—R2-NHCONH—R3  (I),

where R1 and R3 may be the same or different from each other, and represent a C6-30 alkyl group, a C5-8 cycloalkyl group, or a C6-10 aryl group, and R2 represents a divalent aromatic hydrocarbon group having 6 to 15 carbon atoms.

Among the diurea compounds, so-called aliphatic diurea compounds, in which R1 and R3 are the same or different C6-30 alkyl groups, are preferable from the viewpoint of improving the grease fluidity by undergoing shearing in the vicinity of the contact surface. Among these, diurea compounds in which both R1 and R3 are C8 alkyl groups, diurea compounds in which both R1 and R3 are C18 alkyl groups, and diurea compounds in which one of R1 and R3 is a C8 alkyl group and the other is a C18 alkyl are preferable.

The calcium sulfonate complex is a complex (complex soap) containing calcium sulfonate and a calcium salt of an acid other than sulfonic acid.

Examples of the sulfonic acid constituting the calcium sulfonate include dodecylbenzene sulfonic acid, dinonylnaphthalene sulfonic acid, and petroleum sulfonic acid. As the sulfonic acid, dodecylbenzene sulfonic acid is preferable.

Examples of the acid other than the sulfonic acid include 12-hydroxystearic acid and acetic acid.

As the calcium sulfonate complex used in the present invention, a complex formed from dodecylbenzene sulfonic acid, 12-hydroxystearic acid, and acetic acid with a calcium source is particularly preferable. That is, a salt of any two acids selected from the group consisting of dodecylbenzene sulfonic acid, 12-hydroxystearic acid, and acetic acid with calcium is preferable. This complex contains calcium dodecylbenzenesulfonate, calcium 12-hydroxystearate, and calcium acetate. The complex may also contain calcium borate and calcium carbonate.

The amount of the thickener in the grease composition of the present invention is preferably about 2 to 59% by mass, more preferably about 4 to 49% by mass, and particularly preferably about 30 to 49% by mass, based on the total mass of the composition. An amount of about 2% by mass or more is preferable from the viewpoint of keeping the grease in a semi-solid state and preventing the grease from flowing out of the contact portion. An amount of about 59% by mass or less is preferable because the grease is not too hard, an appropriate fluidity is maintained, and application is facilitated.

<Base Oil>

The base oil used in the grease composition of the present invention is preferably a synthetic hydrocarbon oil. Although a base oil other than synthetic hydrocarbon oils can be used, it is preferable to use only a synthetic hydrocarbon oil from the viewpoint of heat resistance, low temperature, and resin compatibility. Examples of the synthetic hydrocarbon oil include poly(α-olefin) (PAO), polybutene, polyethylene, and olefin polymers such as a copolymer of α-olefin and ethylene. A poly(α-olefin) is preferable from the viewpoint that a low viscosity base oil can be selected.

The kinematic viscosity of the base oil at 40° C. is preferably about 200 mm²/s or less. Thereby, appropriate low temperature fluidity can be ensured. The kinematic viscosity of the base oil at 40° C. is preferably about 15 mm²/s or more, and more preferably about 60 to 200 mm²/s.

The content of the base oil in the grease composition of the present invention is an amount usually used for producing grease, and is, for example, about 40 to 97% by mass, and is preferably about 50 to 95% by mass, and more preferably about 55 to 90% by mass, from the viewpoint of allowing the grease to retain a sufficient amount of base oil.

<Organic Sulfur Compound>

The organic sulfur compound in the present invention acts as an anti-wear additive. In general, in an anti-wear additive having sulfur, the anti-wear additive is adsorbed on the friction surface, and the sulfur element reacts with the friction surface to form a wear resistant film. Without wishing to be bound by any theory, it is considered that the organic sulfur compound used in the present invention has no heterocyclic nitrogen in the molecule, and thus is less likely to inhibit the adsorption of sulfur on gold or the gold alloy as the friction surface, and can form a film having effective wear resistance.

Specific examples of the organic sulfur compound used in the present invention include thiols, disulfides, alkylthiophenes, sulfide oils and/or fats, olefin sulfides, alkylthiophosphates, and alkylthiophosphate esters. Thiols, alkylthiophosphates, and alkylthiophosphate esters are preferable, and thiols are particularly preferable.

Examples of the thiol include a thiol having a linear or branched alkyl group having 6 to 24 carbon atoms. A thiol having a linear or branched alkyl group having 14 to 20 carbon atoms is preferable, a thiol having a linear alkyl group having 16 to 20 carbon atoms is more preferable, and a thiol having a linear alkyl group having 18 carbon atoms is most preferable.

Examples of the disulfides include dialkyl disulfides, dibenzyl disulfide, diallyl disulfide, dicyclohexyl disulfide, and diphenyl disulfide. Dibenzyl disulfide is preferable.

Examples of the alkylthiophenes include thiophene in which one or two linear or branched alkyl groups having 1 to 24 carbon atoms are bonded to the thiophene ring. The sulfide oils and/or fats are also called sulfide esters, and refer to sulfides of fatty acid glycerin esters and fatty acid esters. The sulfide oils and/or fats may be active or inactive. Active oils and/or fats are preferable. Note that when the color changes to green or black in the copper strip corrosion test (a test to evaluate whether a copper strip turns green or black after immersing the copper strip in a 5% solution of the additive and allowing it to stand at 100° C. for 3 hours), it is active. A typical compound is represented by the following formulas (1) and (2), where R is a hydrocarbon group in the formula, and x is a number of 1 or more.

The olefin sulfide refers to a sulfide of an olefin, and refers to a compound represented by the general formula R—(S)n-R′, where R and R′ are each independently a linear or branched olefin, and may contain a sulfur atom in the chain, and n is an integer of 1 or more. The olefin sulfide may be active or inactive. Active olefins are preferable. Note that the meaning of the term “active” is the same as that described for the sulfide oils and/or fats. Typical sulfide olefins are represented by the following formulas (3) and (4).

The organic thiophosphoric acid is represented by the formula (RO)_(3-x)R_(x)PS, where R is a hydrocarbon group, and also includes ones in which the RO portion is replaced with RS.

Examples of the alkylthiophosphate esters include compounds represented by the following formula (5).

where R is H, C1-C10 alkyl. R is preferably H or tert-butyl.

The amount of the organic sulfur compound in the grease composition of the present invention is preferably about 0.001 to 5% by mass (sulfur equivalent: about 9.5×10⁻⁵ to 1.3%), more preferably about 0.01 to 5% by mass (about 9.5×10⁻⁴ to 1.3%), and particularly preferably about 0.01 to 2% by mass (about 9.5×10′ to 0.52%), based on the total mass of the composition. When the amount is about 0.001% by mass or more (sulfur equivalent: about 9.5×10⁻⁵% or more), the wear reduction effect of Au plating can be expected. An amount of about 5% by mass or less (sulfur equivalent: about 1.3% or less) is preferable from the viewpoint of obtaining the wear reduction effect at an economically reasonable concentration and preventing the odor caused by a large amount of sulfur compounds.

<Other Additives>

For the grease composition of the present invention, any additive generally used for grease compositions can be used, if necessary. For example, antioxidants, rust inhibitors, metal corrosion inhibitors, oiliness improvers, viscosity index improvers and the like can be mentioned.

Examples of the antioxidants include amine-based, phenol-based, quinoline-based, sulfur-based ones, and zinc dithiophosphate, and amine-based ones are preferable. Examples of the amine-based ones include phenyl-α-naphthylamine, alkylphenyl-α-naphthylamine, and alkyl diphenyl amine, and alkyl diphenyl amine is particularly preferable.

Examples of the rust inhibitors include zinc-based, carboxylic acid-based, carboxylate-based, amine-based, and sulfonate-based ones, and sulfonate-based ones, particularly Ca sulfonate is preferable. Examples of the Ca sulfonate include a calcium salt of petroleum sulfonic acid obtained by sulfonation of aromatic hydrocarbon components in lubricating oil fraction, a calcium salt of synthetic sulfonic acids such as dinonylnaphthalene sulfonic acid and alkylbenzene sulfonic acid, an overbasic calcium salt of petroleum sulfonic acid, and an overbasic calcium salt of alkyl aromatic sulfonic acid, and overbasic Ca sulfonate is particularly preferable.

Examples of the metal corrosion inhibitors include benzimidazole-based and benzotriazole-based ones, and benzotriazole-based ones are preferable. Examples of the benzotriazole-based ones include 1,2,3-benzotriazole, 1,H-benzotriazole, 4-methyl-1,H-benzotriazole, 4-carboxy-1,H-benzotriazole, sodium tolyltriazole, 5-methyl-1,H-benzotriazole, benzotriazole butyl ether, silver benzotriazole, 5-chloro-1,H-benzotriazole, 1-chloro-benzotriazole, 1-di(C8H17)aminomethyl-benzotriazole, 2,3-dihydroxypropyl-benzotriazole, 1,2-dicarboxyethyl-benzotriazole, (C8H17)aminomethyl-benzotriazole, bis(benzotriazole-1-yl-methyl) (C8H17)amine, N,N-bis(2-ethylhexyl)-4-methyl-1H-benzotriazole-1-methyl amine, and N,N-bis(2-ethylhexyl)-5-methyl-1H-benzotriazole-1-methylamine, and 1-[N,N-bis(2-ethylhexyl)aminomethyl]-4-methylbenzotriazole is particularly preferable.

Examples of the oiliness improvers include fatty acids, fatty acid esters, and phosphoric acid esters.

Examples of the viscosity index improvers include polymethacrylate-based, polyisobutylene-based, ethylene-propylene copolymer-based, and styrene-butadiene hydrogenated copolymer-based ones.

The content of these additives is usually about 0.01 to 20% by mass, and preferably about 0.1 to 5% by mass, based on the total amount of the grease composition.

<Penetration>

The penetration of the grease composition of the present invention means a 60-stroke worked penetration. The penetration is preferably about 220 to 400, and more preferably about 265 to 385, from the viewpoint of maintaining an appropriate hardness and achieving both coatability and appropriate fluidity.

<Production Method>

The grease composition of the present invention can be easily produced by mixing the above-mentioned components and additional additives in a desired blending ratio by a conventional method.

<Use>

The use of the grease of the present invention, that is, the type of contact coated with the grease does not matter. For example, sliding contacts, connector contacts, and brush contacts can be mentioned.

Example 1 <Test Grease>

The components used to prepare the grease compositions of Examples and Comparative Examples are as follows.

(Thickener)

-   -   PTFE: Primary particle diameter 120 nm     -   Silica: Specific surface area 170±20 m²/g     -   Aliphatic diurea: Reaction product of diphenylmethane         diisocyanate, octylamine, and octadecylamine     -   Calcium sulfonate complex: A mixture of a Ca salt of each acid,         produced from dodecylbenzene sulfonic acid, 12-hydroxystearic         acid, acetic acid, calcium carbonate, and Ca(OH)₂, with Ca salts         of any two of these acids. This mixture contains calcium borate         and calcium carbonate.     -   Li soap: Lithium 12-hydroxystearate

(Base Oil)

Poly-alpha-olefin: The kinematic viscosities at 40° C. and 100° C. are as presented in Tables 2 and 3. The kinematic viscosity was measured according to JIS K 2220 23.

(Organic Sulfur Compound)

A thiol (stearyl mercaptan), a sulfide (dibenzyl disulfide), a thiophene (octadecylthiophene), an olefin sulfide (active type), a mixture of sulfide oil and/or fat and olefin sulfide (inactive type), thiophosphoric acid, and a thiophosphate ester were used. Table 1 presents the structural formula of each compound.

TABLE 1 Name Molecular Structure Thiol

Disulfide

Thiophene

Olefin Sulfide (Active)

Mixture of Sulfide Oil and/or Fat and Olefin Sulfide (Inactive)

Olefin Sulfide

Sulfide Oil and/or Fat Thiophosphoric Acid

Thiophosphate Ester

Mercaptobenzothiazole (For Comparison)

Dibenzothiazyl Disulfide (For Comparison)

(Additional Additives)

-   -   Antioxidant: Alkyl Diphenyl Amine

The components presented above were used in the amounts in % by mass presented in Table 2 or Table 3 to prepare grease compositions of Examples and Comparative Examples. An alkyl diphenyl amine was added in the compositions in an amount of 2% by mass for Examples and not for Comparative Examples. The balance is the base oil. The worked penetration of the grease composition was unified to 315. The worked penetration was measured by JIS K2220 7.

(Production of Base Grease Whose Thickener is Aliphatic Diurea)

In the base oil, 1 mol of diphenylmethane diisocyanate was reacted at a ratio of 2 mol of a predetermined amine and cooled to obtain a base grease.

(Production of Base Grease Whose Thickener is Li Soap)

Lithium 12-hydroxystearate was stirred in the base oil and then heated to 230° C. Then, the mixture was cooled to 100° C. or lower with stirring to obtain a base grease.

(Production of Grease Composition in which the Thickener is Ca Sulfonate Complex)

Dodecylbenzene sulfonic acid, 12-hydroxystearic acid, acetic acid, calcium carbonate, and calcium hydroxide were stirred in the base oil and heated to 170° C. Then, boric acid and calcium hydroxide were added and further stirred, and then cooled to 100° C. or lower with stirring to obtain a base grease.

An additive was added to each of the base greases obtained above in the ratio presented in Tables 2 and 3, and the test grease composition was prepared by dispersing them with a three-roll mill.

(Production of Grease Composition in which the Thickener is PTFE or Silica)

The test grease compositions were prepared by blending the additives with the base oil in the ratios presented in Tables 2 and 3, then adding PTFE or silica and dispersing them with a three-roll mill.

<Test Method>

Wear resistance was evaluated by a fine sliding test. Specifically, grease was applied to the following flat-plate test piece to a thickness of 120 μm. The flat-plate test piece was driven by reciprocating motion with the embossed test piece pressed from above the grease coating film with a specified load. After sliding at a specified speed a specified number of times, the amount of wear of the embossed test piece was measured with a laser microscope.

[Test Conditions]

Flat-plate test piece: Plate thickness 0.25 mm, Au plating (outermost layer)/Ni plating/Cu base material

Embossed test piece: Plate thickness 0.25 mm, Au plating (outermost layer)/Ni plating/Cu base material

Temperature: 25° C.

Load: 4.9 N

Speed: 0.1 mm/s

Sliding width: 0.3 mm

Number of slides: 1000 cycles

Grease coating thickness: 120 μm

Evaluation: Wear amount 22000 μm³ or less . . . O (acceptable)

-   -   Wear amount over 22000 μm³ . . . ×(unacceptable)

TABLE 2 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Thickener PTFE PTFE PTFE PTFE PTFE PTFE % by Mass 35 35 35 35 35 35 Base Oil PAO PAO PAO PAO PAO PAO Kinematic Viscosity 107 107 107 107 107 107 of Base Oil at 40° C. Organic Sulfur Thiol Sulfide Thiophene Olefin Mixture of Sulfide Thiophosphoric Compound Sulfide Oil and/or Fat and Acid (Active) Olefin Sulfide (Inactive) % by Mass 2 2 2 2 2 2 Wear Amount [μm³] 9870 15700 19900 18300 15600 13300 Determination ◯ ◯ ◯ ◯ ◯ ◯ Comparative Comparative Example 7 Example 8 Example 1 Example 2 Thickener PTFE PTFE PTFE PTFE % by Mass 35 35 35 35 Base Oil PAO PAO PAO PAO Kinematic Viscosity 107 107 107 107 of Base Oil at 40° C. Organic Sulfur Thiophosphate Thiol Mercapto- Dibenzothiazyl Compound Ester benzothiazole Disulfide % by Mass 2 0.02 2 2 Wear Amount [μm³] 12300 17100 28300 33500 Determination ◯ ◯ X X

TABLE 3 Comparative Comparative Comparative Comparative Comparative Example 9 Example 10 Example 11 Example 12 Example 3 Example 4 Example 5 Example 6 Example 7 Thickener Silica Aliphatic Ca Sulfonate Li Soap PTFE Silica Aliphatic Ca Sulfonate Li Soap Diurea Complex Diurea Complex % by Mass 11.5 8.5 27 14.5 35 11.5 8.5 27 14.5 Base Oil PAO PAO PAO PAO PAO PAO PAO PAO PAO Kinematic Viscosity 107 107 107 107 107 107 107 107 107 of Base Oil at 40° C. Organic Sulfur Thiol Thiol Thiol Thiol None None None None None Compound % by Mass 2 2 2 2 0 0 0 0 0 Wear Amount [μm³] 9770 16300 16500 8890 41500 43900 53500 36600 22300 Determination ◯ ◯ ◯ ◯ X X X X X

Examples 1 to 7, which used at least one selected from thiols, sulfides, thiophenes, sulfide oils and/or fats olefin (active type), mixture of sulfide oils and/or fats and sulfide olefin (inactive type), thiophosphoric acid, and thiophosphate esters as an additive, are superior in wear resistance as compared with Comparative Examples 1 and 2.

This improvement is observed in all of PTFE, silica, aliphatic diurea, calcium sulfonate complex, Li soap, urea-based thickener, and PTFE (Examples 1, 8 to 11). Therefore, by using the organic sulfur compound specified in the present invention, it is possible to improve the wear resistance of the electrical contact made of gold or a gold alloy regardless of the thickener and the base oil. 

1. A grease composition for electrical contacts, comprising a thickener, a base oil, and an organic sulfur compound, wherein the organic sulfur compound is a compound having no heterocycle containing nitrogen in a molecule thereof.
 2. The grease composition according to claim 1, wherein the organic sulfur compound is at least one selected from the group consisting of thiols, disulfides, alkylthiophenes, sulfides of oils or fats, sulfides of olefins, and thiophosphoric acid or esters thereof.
 3. The grease composition according to claim 1, wherein the organic sulfur compound is at least one selected from the group consisting of thiols.
 4. The grease composition according to claim 1, wherein the organic sulfur compound is a thiol having a linear or branched alkyl group having 6 to 24 carbon atoms.
 5. The grease composition according to claim 1, wherein the organic sulfur compound is stearyl mercaptan.
 6. The grease composition according to claim 1, wherein the organic sulfur compound is contained in an amount of about 0.001 to 5% by mass based on the total mass of the composition.
 7. The grease composition according to claim 1, wherein the base oil is a synthetic hydrocarbon oil.
 8. The grease composition according to claim 1, wherein the base oil is poly-alpha-olefin.
 9. The grease composition according to claim 1, wherein a kinematic viscosity of the base oil at 40° C. is about 200 mm²/s or less.
 10. The grease composition according to claim 1, wherein the base oil is contained in an amount of about 40 to 97% by mass based on the total mass of the composition.
 11. The grease composition according to claim 1, wherein the thickener is at least one selected from the group consisting of Li soaps, urea compounds, polytetrafluoroethylene, inorganic fine particles, and calcium sulfonate complex.
 12. The grease composition according to claim 1, wherein the thickener is polytetrafluoroethylene.
 13. The grease composition according to claim 1, wherein the thickener has a primary particle diameter of about 80 to 400 nm.
 14. The grease composition according to claim 1, wherein the thickener is contained in an amount of about 2 to 59% by mass based on the total mass of the composition.
 15. The grease composition according to claim 1, further comprising at least one additives selected from the group consisting of antioxidants, rust inhibitors, metal corrosion inhibitors, oiliness improvers, and viscosity index improvers.
 16. The grease composition according to claim 15, wherein the additive is contained in an amount of about 0.01 to 20% by mass based on the total mass of the composition.
 17. The grease composition according to claim 1, wherein the composition has a 60-stroke worked penetration measured according to JIS K2220 7 of about 220 to
 400. 18. The grease composition according to claim 1, wherein a material of the electrical contacts is gold or a gold alloy. 